There are two primary objectives of the proposed research: Evaluation of the relative contributions of bradycardia, peripheral vasoconstriction, and tolerance to hypoxia, to the duration of breath-hold in semi-aquatic and terrestrial mammals. Evaluation of possible detrimental effects of diving (oxygen concerving) reflexes evoked by various stimuli to the nares (water, air stream, smoke) upon animals that were previously made to develop chronic congestive heart failure. Three species of mammals will be employed in these studies: a) Beaver-like mammals called Nutrias which are semi-aquatic or diving mammals. b) Dogs and rabbits - which may be considered as basically terrestrial or non-diving mammals.